Research And Application Of High Speed Copper Electroplating For The Interconnection Micro-holes Of Printed Circuit Board

Rapidly developing high-tech information technologies such as artificial intelligence and 5G have penetrated into our daily life.These high-techs inevitably demand better manufacturing technologies and processes,among which is the manufacturing technology of printed circuit board(PCB),the mother of information products.The trend of PCB is miniaturization,functionalization,high integration and high stability.The metallization of the micro-vias and through-holes for interconnecting the layers of multilayer PCBs is one of the key factors affecting the development of PCBs.The yield and reliability of PCBs are highly dependent on the properties of the plating layers.Therefore,the current industry usually employs low plating current density to ensure the reliability of deposited copper layer,for instance,the current densities of plated through hole and plated microvia are lower than 3 A/dm2 and 2 A/dm2 respectively meanwhile the temperature is lower than 30 °C.The mechanism of the additives in copper plating has not been thoroughly understood.As a result,it is impossible to tell the effect on the additives after changing the plating conditions and how to choose a suitable additive system.However,under the continuous promotion of increasing yield from industries,it is an inevitable trend to increase the plating current density.Therefore,a thorough understanding of the mechanism of the additives in copper plating is highly desired.In this dissertation,density functional theory(DFT)calculations,molecular dynamic(MD)simulations,electrochemical measurements and electroplating are employed to investigate in detail the mechanism of the additives in copper plating.High-speed copper plating is successfully achieved by additive optimization and temperature increase.The results bring better economic and social benefits,and provide a very important theoretical basis for the future research on the development of electroplating.The main research is as follows.(1)The electronic structure parameters of typical additives in copper plating were calculated by DFT,the additives are: accelerators,namely mercaptopropane sulfonic acid(MPS)and bis-(acid-sulfopropyl)-disulfide(SPS),inhibitor,namely polyethylene glycol(PEG),leveler,namely janus green b(JGB).MDs were employed to simulate the adsorption process of these four additives on copper surface at different temperatures.The basic quantum chemical properties and the important functional groups of these additives were obtained by DFT.In addition,it is demonstrated that MPS is a strong metal ligand and the sulfur group of SPS is unstable,PEG is chemically stable,JGB has the highest molecular reactivity.The adsorption configuration and adsorption energies of the additives on copper surface were obtained by MDs.The results show that MPS and SPS have weaker adsorption,while JGB has the strongest adsorption.Meanwhile,increasing the temperature weakens the adsorption of MPS and SPS while enhancing the adsorption of PEG and JGB.And increasing the temperature changes the adsorption configuration of MPS and SPS.These theoretical data lay a theoretical foundation for further study of their mechanism and explanation of related experimental phenomena.(2)The acceleration mechanism of MPS in copper plating proposed by Dow is confirmed and improved.DFT were employed to calculate the configurations of the accelerating intermediate MPS-Cu(I)-Cl-Cu(II)after obtaining different numbers of electrons.The results show that Cu(II)bonded to the sulfonate of MPS would be reduced to Cu(0)after the intermediate gets two electrons.This is the core of the accelerating mechanism of MPS.Meanwhile,the bond between the thiol of MPS and Cu(I)is strong despite of obtaining two electrons.It is predicted that the MPS intermediates desorb from the surface after the reduction of Cu(II)by MD.Subsequent electrochemical experiments and electroplating experiments confirmed the electrochemical stability of MPS-Cu(I)during copper electro-deposition,and when Cu(II)was reduced,the intermediates would desorb into the solution and continue to accelerate the reduction of Cu(II).This complete mechanism can reasonably explain most of the experimental phenomena in copper plating,such as the accumulation of thiol copper complexes at the cathode in rotating ring disk electrode experiments.(3)The leveling mechanism of two typical levelers,namely JGB and polymerizates of imidazole and epichlorohydrin(IMEP),were compared by theoretical calculations and experiments.We proposed that a leveler with high ESP is favorable for high-speed plating meanwhile achieved high-speed copper plating with a high ESP leveler as the core of the additive system.Together with electrochemical measurements,theoretical calculations show that IMEP can result in stronger cathode polarization than JGB while MPS has both synergistic and antagonistic effect with IMEP and has an antagonistic effect with JGB.In addition,the accelerating effect of MPS was eliminated by JGB at high current density.The final electroplating experiments directly confirmed that IMEP with high ESP value is favorable for high-speed copper plating: for through-holes with depth and diameter of 160 mm and 0.25 mm,respectively,raising the current density from 1 A/dm2 to 3 A/dm2,the throwing power(TP)of adopting the two levelers was reduced from 84.2% to 74.4%(IMEP)and 86.2% to 63.7%(JGB),which are in accordance with the results of theoretical calculations.After that,IMEP and copolymers of imidazole and 1,4-butandiol diglycidyl ether(IBDGE)with high ESP were employed as the leveler for copper plating at higher current density respectively.For instance,the TP value maintained at around 55% at the current density of 6 A/dm2.In addition,the reason for the appearance of the overhang and thin knee in copper plating is the interaction between the levelers and accelerators by combining the results of theoretical calculations and plating experiments.This has great significance for practical industrial electroplating.(4)The effect of temperature on copper plating process and the additive system was investigated in detail.High-speed copper plating was successfully achieved by means of increasing the temperature,which fills up the insufficiency of copper plating in this aspect.Electrochemical measurements show that the limited current density is proportional to the temperature,proving that it is reasonable to increase the current density by raising temperature.In addition,raising temperature brought about the following effects on the additive system:(1)the polarization effect of Cl-and PEG was weakened,(2)the depolarization effect of MPS and SPS was weakened in diluted Cu2+ solution but was enhanced in concentrated Cu2+ solution,(3)the polarization effect of IMEP was enhanced,(4)the polarization effect of JGB was irregular.Suitable electrolytes for high-speed micro-via filling were screened by electrochemical experiments.Under the conditions of 4 A/dm2 and 35 °C(the electrolyte with SPS)or 50 °C(the electrolyte with MPS),for micro-via with depth and diameter of 70 ?m and 100 ?m,the micro-via can be filled up within 20 min.The filling time was shortened by 30% as compared to current industrial micro-via filling.One suitable plating solution for high-speed plated through-holes was developed by optimization experiments.For through-holes with depth and diameter of 160 mm and 0.25 mm,respectively,exceed 80% TP was achieved under the conditions of 4.3 A/dm2 and 35 °C.Meanwhile,the change of the amount of the additives at high temperature was explained according to the results of theoretical calculations.Finally,the problem of the brightness of the copper surface at high temperature plating condition is solved.And the reason for that was analyzed meanwhile achieving the plated throughholes at higher temperature.(5)The effect of raising temperature on the PEG inhibitor structure was investigated by means of DFT calculations and experiments.The results of DFT calculations indicated that the bond between Cu(I)and the oxygen atom in PEG-Cu(I)-Cl structure was weakened and the adsorption of Cl-by raising temperature leaded to unstable Cu(I).This is the dominant reason for the weakened polarization effect of PEG at high temperature.Subsequent electrochemical experiments and copper plating experiments validated this deduction.The feasibility of replacing Cl-with Br-for high-speed copper plating was investigated.The results of DFT calculations indicated that the effect of raising temperature on PEG-Cu(I)-Br inhibitor structure and the adsorption of Br-on copper surface is slight.Electrochemical experiments indicated that high-speed micro-via filling can be achieved by using the plating solution with Br-.The final copper plating experiments validated that the micro-via was filled up within 20 min under the condition of 4 A/dm2 and 50 °C for micro-via with depth and diameter of 70 ?m and 100 ?m.The filling time was shortened by more than 40%.